Thanks to ComScore, which provided us data on the top most popular Web domains in the world.

Amazon.com - 163 Million Unique Visitors

What it is: The ultimate online shopping destination for electronics, apparel, sporting goods, and even food.

How it got so big: When Amazon first launched, it was nothing more than a few people packing and shipping boxes of books. But it has since grown to shipping everything from trumpets to the Kindle Fire.

Sina.com.cn - 169 Million Unique Visitors

What it is: Chinese mobile portal for media and user-generated content.

How it got so big: In the early 2000s, Sina was known as the "Yahoo of China." Sina launched microblogging service Weibo in 2009, and has grown to more than 400 million users.

WordPress.com - 170.9 Million Unique Visitors

What it is: Blogging platform.

How it got so big: WordPress has been able to attract users by offering dead-simple tools for blogging and web publishing. Given that it's open source, WordPress has the upper hand on other platforms that require licensing fees. Back in 2004, when Movable Type changed its license, a lot of people left for WordPress.